The present invention relates generally to wave filters, and deals more specifically with radio frequency filters of the kind comprising two or more coaxial, dielectric resonators in juxtaposition. The radio frequency filters according to the invention find typical applications in mobile or portable telephone sets, although no unnecessary limitations thereto are intended.
Bandpass or bandstop radio frequency filters have been known which typically take the form of a pair of juxtaposed coaxial dielectric resonators operating in transverse electromagnetic (TEM) mode, as disclosed for example in U.S. Pat. No. 5,578,975 to Kazama et al. Methods have also been known of capacitively coupling together the dielectric resonators. One such known method, according to Japanese Unexamined Pat. Pub. No. 7-176911, teaches to provide layers of electrically conductive material on the opposed surfaces of the resonators, thereby obtaining capacitances between the conductive layers and the inner conductors of the resonators, and to solder or otherwise join the conductive layers. These conductive layers are referred to as resonator coupling conductors.
The dielectric resonators have been further provided with additional conductor layers on their outer surfaces for use as terminals in connecting the filter to external circuits. These terminals should of course be electrically isolated from each other as much as possible. Difficulties have been encountered, however, in realizing a desired degree of isolation between the terminals because they have been capacitively coupled together in devices composed of juxtapositions of two or more dielectric resonators.
Additional problems left unsolved with dielectric resonator wave filters arise from the presentday demand for smaller and smaller devices. The outer conductor of each resonator is provided with extensions to one end of the dielectric body according to one known downsizing method, and, according to another such method, the inner conductor is joined directly to a conductive layer, or inner conductor extension, formed on one end of the dielectric body.
Such known downsizing methods are alike in aiming at lower resonance frequencies with each dielectric body maintained at the same length as before. This objective, known as the wavelength shortening effect, is obtained as the capacitance between the extensions of the outer conductor and the inner conductor, or between the extension of the inner conductor and the outer conductor, of each resonator is connected in parallel with the resonance circuit of each resonator proper, resulting in a decrease in resonance frequency. For example, the resonance frequency of a device having a pair of dielectric resonators may decrease from 1900 megahertz, in the case where no such downsizing measures are taken, to as low as 1000 megahertz when the noted capacitance additionally connected in parallel with the resonance circuit of each resonator is 20 picofarads.
Let us now consider a wave filter comprised of juxtaposed dielectric resonators having the inner conductor extensions, the terminal conductors, and the resonator coupling conductors. Capacitances between resonator coupling conductors and inner conductors and capacitances between terminal conductors and inner conductors change with the size of the inner conductor extensions. The aforesaid wavelength shortening effect is therefore not adjustable by the inner conductor extensions without affecting the capacitances in question.
The terminal conductors and the resonator coupling conductors have presented a further problem. These conductors have been required to be of not less than a certain size for providing the desired capacitances, running counter to the size reduction of the filters incorporating them.
A yet further problem with dielectric filters in general has been the spurious resonance at thrice the fundamental frequency or thereabouts. The spurious resonance has resulted in insufficient attenuation of that higher harmonic.